A close relationship between desensitization and internalization of mu opioid receptors has been proposed based on differential actions of series of agonists. A wide variety of opioids that have dramatically different pharmacological properties are used in the clinic for the treatment of various forms of pain as well as the managment of drug addiction. Agonists that are commonly administered on a chronic basis are morphine, methadone and buprenorphine. Morphine and buprenorphine do not induce marked acute desensitization or trafficking of receptors, but, methadone is efficient at causing both processes. The role that desensitization and internalization have in the development of tolerance and dependence to opioids has been a controversial subject and has been studied in a variety of model systems. There has, however, been only limited study on potential differences in the development and expression of tolerance that result from the chronic treatment of animals with these three commonly used opioid agonists. The goal of this work is to examine both desensitization and internlaization of receptors in neurons after treating animals chronically with each of these agonists. Opioid receptor desensitization and internalization are highly regulated processes in neurons and the primary hypothesis of this proposal is that these processes are altered in an agonist specific way following chronic treatment. The first aim will examine tolerance and the recovery from acute desensitization in animals (rats) treated chronically with morphine, methadone and buprenorphine. The second aim will examine how receptor internalization induced by a desensitizing concentration of agonist is altered following the chronic treatment of animals (transgenic mice) with each of the three agonists. The third aim will test the hypothesis that receptor desensitization and internlaization are separate events and that receptor internalization and functional reinsertion to the plasma membrane is dramatically altered by chronic agonist treatment. By gaining knowledge of the processes that are selectively affected by some agonists and not others, the mechanisms underlying the development of tolerance and dependence may be identified and applied to more effective treatment of chronic pain and addiction.